Data-dependent non-double-couple components of shallow earthquake source mechanisms: Effects of waveform inversion instability

Seismic moment tensors are routinely determined for global earthquakes using waveform inversions, and the resulting double‐couple and non‐double‐couple characteristics are extensively used in tectonic analyses. For shallow events, several moment tensor components are intrinsically poorly resolved, especially for surface wave inversions, which can lead to artificial non‐double‐couple components or can obscure actual non‐double‐couple components in the source representations. The relatively greater instability of surface wave inversions can cause differences between inversions based on only body waves and joint inversions of body and surface waves. The Harvard CMT catalog exhibits systematic variations in non‐double‐couple components with seismic moment, which can be attributed to use of only body waves in inversions for small events and combined body and surface waves for large events. Systematic behavior of non‐double‐couple components with regional strain environment is significantly less dependent on seismic moment when solutions influenced by less stable surface waves are omitted.